Computed tomography (CT) imaging systems operate by projecting an x-ray beam from an x-ray source through an attenuating object, such as a patient. The attenuated x-ray beam is then detected by a detector assembly. Some materials, such as bone, are much more x-ray attenuating than other materials, such as soft tissue. Conventional third-generation CT imaging systems acquire attenuation data by projecting a polychromatic x-ray beam from the x-ray source. The polychromatic x-ray beam contains many different frequencies of x-rays and is typically centered around a particular energy level. With sufficient angular coverage around the patient, cross-sectional images can be formed revealing the inner structure of the scanned object. The images are typically displayed on a flat-screen monitor or cathode ray tube. A virtual 3-D image may also be produced based on data acquired during a computed tomography scan.
However, some materials share very similar x-ray attenuation characteristics at a particular energy level. For example, bone and iodine contrast agent have similar x-ray attenuation characteristics at some commonly used energy levels. As a result, it can be difficult to differentiate materials with similar x-ray attenuation characteristics at a particular energy level. Recently, multiple-energy CT imaging systems have been developed. By collecting x-ray attenuation data at more than one energy level, it is possible to gain additional insight into the nature of the scanned object.
In a conventional third-generation dual-energy CT imaging system, a processor may rapidly switch the output from a generator so that the input voltage to the x-ray source changes from projection to projection. For example, a typical dual-energy CT imaging system may rapidly alternate between acquiring a high-energy projection and a low-energy projection. In order for a conventional dual-energy CT imaging system to work well, the time spent transitioning between the high-energy level and the low-energy level must be short relative to the duration of each projection. Otherwise, the effective energy level during the high-energy projections will be less than the desired high-energy setting and the effective energy level during the low-energy projections will be greater than the desired low-energy setting. Given the fact that the sampling rate for a state-of-the-art third-generation CT imaging system is several kilohertz, the generator must transition between the high-energy level and the low-energy level in significantly less than a fraction of a millisecond. Enabling a generator to switch this quickly places an enormous demand on the generator hardware. Predictably, the demands on hardware will further increase as scan speed increases.
For these and other reasons, there is a need for a multiple-energy CT imaging method and system that significantly reduces the demand on generator hardware without negatively affecting scan time or image quality.